An oriented electrical steel sheet contains 3.1% of a Si component and has a texture in which an orientation of grains is a {110}<001> direction, and because the product has an excellent magnetic characteristic in a rolling direction, the product is used as an iron core material of a transformer, a motor, a generator, and other electrical devices using the characteristic.
Recently, while an oriented electrical steel sheet of a high magnetic flux density is commercially available, a material having small iron loss has been requested. In an electrical steel sheet, iron loss may be enhanced with four technical methods including a first method of accurately orienting a {110}<001> grain direction of a magnetic easy axis of an oriented electrical steel sheet in a rolling direction, a second method of forming a material in a thin thickness, a third method of minutely forming a magnetic domain through a chemical and physical method, and a fourth method of enhancing a surface property or imparting surface tension by a chemical method such as surface processing.
Excellent insulating coating in an oriented electrical steel sheet should generally have a uniform color that does not have a defect in an external appearance, but by adding several technologies that impart a function, technology that enhances an electrical insulating property and that reinforces a close contacting property of a film is generally used.
However, currently, while a request for a low iron loss oriented electrical steel sheet increases, it is requested that a final insulating film has high tension, and it has been determined that an actual high tension insulating film largely contributes to magnetic characteristic enhancement of a final product.
In order to improve a characteristic of a tension film, a control technique of several process factors has been applied, and an oriented electrical steel sheet presently available as a product obtains an iron loss reduction effect by adding a tension stress to a steel sheet by using a difference of a thermal expansion coefficient of an insulating film that is formed on a forsterite (Mg2SiO4, hereinafter, base coating)-based base film and a steel sheet.
As a representative insulating film forming method, in Japanese Unexamined Patent Application No. H11-71683, a method of improving film tension using colloidal silica having a glass transition point of a high temperature is disclosed, or in Japanese Patent No. 3098691 and Japanese Patent No. 2688147, a technology that forms an oxide film with high tension in an electrical steel sheet using alumina sol of an alumina subject and a boric acid mixture liquid is suggested.
Further, by actively enhancing a property of an oriented electrical steel sheet surface, magnetism of a material may be enhanced, and by removing an oxidation layer that is inevitably generated in a decarburization annealing process among a process and a base coating layer that is generated through a chemical reaction of a MgO slurry that is used as a fusion-bonding inhibitor of a coil, an object thereof can be achieved.
Technology that removes the base coating includes a method of forcibly removing a product in which base coating is already formed like a common material with sulfuric acid or hydrochloric acid, and this is disclosed in Japanese Patent No. 1985-076603.
However, in such a case, a complex process such as chemical polishing or electrolytic polishing is required, and particularly, in order to remove a surface with a constant thickness, there is a difficulty that an acid concentration in a process should be constantly maintained and a processing cost offsets a performance improvement effect of a product.
Further, when surface roughness of an obtained product is excessively smooth, insulating coating cannot be performed on the product, and thus a close contacting property may not be secured and an insulating property is very poor without using a physical/chemical deposition method.
In order to overcome such a technical limitation, in a process of generating a base coating, technology (hereinafter, glassless technology) that removes or suppresses the base coating was suggested (U.S. Pat. No. 4,543,134) and was performed in two directions of technology that adds a chloride to MgO, which is an annealing separating agent, and that uses a surface etching effect in a high temperature annealing process, and technology that does not form a base coating in a high temperature annealing process by applying Al2O3 powder as an annealing separating agent.
First, in glassless technology, technology that does not form a base coating using Al2O3 powder performs a process of (decarburization annealing)—(acid pickling)—(Al2O3 application)—(high temperature annealing)—(forming of oxide film by preliminary annealing)—(tension film coating), and is a method using a property in which Al2O3 does not react with an oxide layer existing at a material surface.
However, in the technology, Al2O3 that is used as an annealing separating agent should be very small and uniform in a powder form, but when producing an industrial use powder in a slurry for application having a grain size of about 2-10 μm, it is difficult to maintain the powder in a distribution state.
As another glassless technology, a method of removing a base coating includes a chloride addition method and performs a process of (decarburization annealing)—(MgO+chloride powder application)—(high temperature annealing)—(acid pickling)—(tension film coating), and has almost the same process as a common production method.
As in U.S. Pat. No. 4,875,947, a representative chloride addition method is technology that uses a fusion-bonding inhibitor, i.e., an annealing separating agent, between coil plates as a main component upon annealing MgO at a high temperature, and that forms an FeCl2 film by enabling a chloride to react with a material surface while high temperature annealing by adding the chloride (hereinafter, conventional glassless additive) such as one based on Ca, Li, K, Na, and Ba to the annealing separating agent and prevents a glass film layer from being formed by removing the FeCl2 film by evaporation at a surface.
However, according to the technology, an oxide film having excellent application workability but still having a thin thickness exists, and obtained surface roughness is higher than that of a specimen that is produced by chemical polishing and thus only effects advantageous in workability, i.e., punching of a product due to a base coating member rather than an iron loss enhancement effect, may be expected.
Therefore, technology that can compensate this was suggested, and as described in Japanese Patent No. 1993-167164, a smoothed product having excellent roughness compared to that of an existing annealing separating agent using BiCl3 as the chloride and having no residual material, compared with a general chloride, was obtained, and has excellent iron loss compared to that of a common product that forms a base coating.
However, in order to use MgO and BiCl3 that are used in the technology as an annealing separating agent, when MgO and BiCl3 are produced in a slurry phase together with water, as suggested by a spinel (Al2O3.MgO) by a reaction with active MgO and an Al component existing in steel, it is difficult to obtain a product having very low roughness and Fe oxide generation that is caused by dissociation of BiCl3, which is together used chloride is accelerated and thus after high temperature annealing, a, Fe-based residual material remains at a material surface.
Due to the problem, it is very difficult to obtain an excellent product in terms of iron loss compared to that of an oriented electrical steel sheet general material and in which the base coating is excluded.